The goal of this research is to define the morphological changes that accompany migration and differentiation of neuroblasts and which provide a basis for understanding the factors controlling these events. Anatomical analyses will be directed to the formation of the auditory nuclei, starting with the magnocellular nucleus and the cochlear duct, in the medulla oblongata of chick embryos staged according to Hamburger and Hamilton. The Golgi method and Nissl, reduced silver, and silver-degeneration techniques will be used to identify the migratory and post migratory neuroblasts, the stages in the differentiation of the sensory neurons, including formation of their characteristic dendrites and axons, and the synaptic endings of the auditory nerve fibers at the light microscopic level. Electron microscopy will be applied to comparably staged material in order to describe the fine structure of the developmental changes, especially the formation of synapses and other junctions between the afferent axons and the second-order neurons and hair cells. The influence of the auditory nerve axons and their synaptic structures on the migration and differentiation of the magnocellular neurons will be studied by observations on the effects of otocyst ablation at different developmental stages on the temporal sequences of morphological changes defined at the light and electron microscopic levels. The findings should help to clarify the possible interactions between the peripheral sensory end-organ and the central nervous system in the early development of the auditory system and lead to specific hypotheses on the mechanisms for the formation of sensory neurons and their synaptic connections.